Mechanisms of demyelination are studied in demyelinating diseases caused by temperature sensitive mutants of mouse hepatitis virus, picornavirus and Marek's disease virus. Lesions induced by these viruses are examined by electron microscopy both in vivo and in vitro in organ cultures of central and peripheral nervous tissues. Particular attention is directed to interaction of virus as well as lymphoid cells and humoral factors with myelin lamellae or the plasma membranes of myelin supporting oligodendrocytes and Schwann cells. Sera and lymphoid cells obtained from experimental animals will be tested for demyelinating activity in vitro during various stages of the disease.